Dielectric, electro-optic and spectroscopic properties of (η 6-arene)M(CO) 3 complexes (M = Cr, Mo or W): a study of metal—arene interactions

Abstract

Electric dipole moments, optical polarisabilities, infrared, 1H and 95Mo NMR spectra of a number of (η 6-arene)M(CO) 3 complexes, where M is Cr, Mo or W, were examined to provide information on metal—arene interactions and on intramolecular electronic transmissions. It was shown that greater electron displacements from the arene to the metal occur with the heavier metals, but this is largely localised in the metal—arene electron system with the carbonyl ligands little affected by change of metal M. Coordinative interaction between the arene and M(CO) 3 fragments results in a metal—arene moiety with very high polarisability along the bond axis and relatively free electron transmission in that direction. The observations also suggest strong π-, and possibly δ-, character for the metal—arene bond and magnetic anisotropy of the electron system with the maximum susceptibility perpendicular to the M—arene bond axis. Further evidence for π-bonding is provided from 95Mo NMR studies of (η 6-arene)Mo(CO) 3 complexes. Polarisability data and ortho-benzylic 1H NMR couplings are interpreted to show decreased aromaticity in the arenes on complex formation with M(CO) 3; from the latter, it appears that ring current depletion is greatest for complexes of the heavier metals.